1. Field of the Invention
The present invention is directed to a method for automatically controlling the exposure during a CT (computed tomography) scan, as well as to an apparatus for automatically controlling the exposure in CT scanning.
2. Description of the Prior Art
Computed tomography systems of the last generation have enhanced the ability to perform large volume scans over a significant region of the body of an examination subject. This capability has improved the efficiency of CT installations, by shortening the examination time and thereby increasing the patient throughput. During relatively long examinations, such as spiral (helical) examinations, which cover a large volume of the body of the subject, the radiation absorption by the patient changes significantly along the spiral path. Without undertaking additional measures, however, the CT system would employ X-rays of a constant intensity along the entirety of the scanned volume. When the shoulder-chest region of the patient is being scanned, for example, the radiation absorption by the patient in the shoulder slices is very high, thereby necessitating a high X-ray intensity in order to obtain low noise images. When scanning the lungs, however, at a different slice location along the longitudinal length of the body, this X-ray intensity may be excessive, because the lungs have a very low density and thus exhibit weak X-ray absorption.
It is highly desirable to have good quality images with a reasonable X-ray exposure, and therefore a need has been recognized for automatic exposure control in these types of relatively long duration scans. Such automatic exposure control is for the purpose of adjusting the X-ray intensity in each slice, according to the patient absorption which prevails in the body region for which the region is being obtained.
In conventional radiographic imaging, automatic exposure control is a standard feature that avoids the use of an insufficient X-ray dose, or a higher then necessary X-ray dose. Many users of CT systems are requesting automatic exposure control in such systems in order to reduce the total dose to which a patient is exposed.
Several methods are known for implementing automatic exposure control during long-duration CT scans, i.e., scans consisting of many rotations around the body of the patient, wherein many slice image are generated. Three basic approaches have conventionally been employed for this purpose.
One of these approaches is based on information acquired prior to a main scan, i.e., information acquired in a so-called scout scan. This technique is exemplified by U.S. Pat. No. 5,400,387. In this known technique, two orthogonal scout (topographic) scans are conducted over the entirety of the body segment that is to be subsequently diagnostically scanned. Based on the attenuation information obtained by these scout scans, and dependent on target pixel noise, the system calculates the appropriate tube current for the X-ray tube for each slice in the main scan. This method has the disadvantage of requiring an extra radiation dose to be applied to the patient during the scout scans. This known technique also necessitates a longer overall examination time, thus reducing the throughput of the CT installation. Moreover, the information provided by the scout scans may not be sufficient to allow the appropriate tube current to be calculated, if the scout scans, for example, failed to identify the maximum or minimum patient absorption for each slice. Usually the scout scans are performed so as to obtain a lateral view or an anterior-posterior view, but it is known that for some body regions, such as the abdomen region, the maximum and minimum patient absorption are best identified at other (different) viewing angles. Moreover, in general terms the use of only two orthogonal views is insufficient to completely define the patient absorption profile around a complete slice, as is required for a fully accurate calculation of the tube current. Moreover, this known technique also will produce inaccurate results, if the patient""s body moves between the scout scans and the main scan.
A further known technique is to undertake automatic exposure control using a negative feedback loop, which includes the X-ray tube and the voltage and current generator which feeds the X-ray tube, as well as one or more detectors. This technique is described in U.S. Pat. No. 5,696,807. In this known technique, a feedback controller measures an average signal from an X-ray detector (conventionally called photon saturation), and adjusts the tube current in order to minimize any deviation between a predetermined, desired signal level and the actual signal level. The predetermined signal level is calculated in advance, in order to achieve a specified target noise. A disadvantage of this known technique is that it has limited practical application, since it is well-known that the X-ray beam emitted by a conventional X-ray tube does not become modified sufficiently quickly, due to the thermal inertia of the heating system of the X-ray tube, to allow tracking of rapid changes in the tube current. In other words, after a change in the tube current is made, there is a time delay until X-rays are actually emitted at a level corresponding to the changed current. It is also known that the patient absorption profile exhibits rapid changes as the tube rotates around the patient. A negative feedback system, which would modulate the tube current following the patient absorption profile necessitates a higher modulation speed of the X-ray tube, and thus cannot be implemented with conventional CT X-ray tubes. The remedy of slowing down the response time of the feedback controller, in order to accommodate the slow modulation capability of the X-ray tube, usually will cause problems in the overall system response. In such a situation, the radiation profile being currently employed will not match the actual patient absorption profile, thereby resulting either in an increased image noise or an inefficient X-ray dose.
A third known technique is to undertake exposure control based on statistical data and user entries, as described in U.S. Pat. No. 6,094,468. In this method, the appropriate tube current is calculated based on statistical data regarding the expected patient absorption within various anatomical body regions, and/or based on geometric measurements of the patient""s size and dimensions, taking corrective inputs made by an operator into account. Therefore, this technique cannot be classified as a truly automatic exposure control, but instead is a pre-programmed modulation of the dose profile, and therefore this method may not necessarily achieve the best results. This technique is dependent on the prediction accuracy of the patient absorption, which may vary significantly from patient-to-patient. This system also is dependent on the skill and knowledge of the operator in deciding on and entering the appropriate corrective inputs.
It is an object of the present invention to provide a method and an apparatus for undertaking automatic exposure control in a CT scan, wherein the aforementioned disadvantages of known systems and techniques are avoided or at least minimized.
This object is achieved in accordance with the principles of the present invention in a computed tomography apparatus, and in a method for operating a computed tomography apparatus, wherein at least the focus of an X-ray source is rotated through at least one revolution around an examination subject for conducting a CT scan of the subject, and wherein a control arrangement is connected to a radiation detector, on which an X-ray beam emitted by the X-ray source is incident after being attenuated by an examination subject, and to the X-ray source, and wherein the control arrangement, during a first half of the revolution, calculates an attenuation profile of a portion of the subject from electrical signals generated by the radiation detector in the first half of the revolution, and calculates an extrapolated attenuation profile for a second half of the revolution from the actual attenuation profile, and adjusts at least one operating parameter of the X-ray source during the second half of the revolution dependent on the extrapolated attenuation profile. The adjustment of at least one parameter of the X-ray source in the second half of the revolution can be undertaken with the goal of achieving a target pixel noise in each slice image. This target noise may be different for each body region or for each examination protocol. The respective target noise values for different body regions and/or different examination protocols can be stored in a memory in default tables, or can be selectively specified by an operator of the CT system through an interface. If a relatively long spiral or sequential CT scan is to be undertaken, different target noise values can be set for different slice locations along the longitudinal length of the scan, dependent on the different attenuation profiles of the body regions which are successively scanned. Thus, for each body region, i.e. for each slice image, an appropriate X-ray dose is employed in order to generate a good quality image, while not exposing the examination subject to an unnecessarily high radiation dose when this is not needed for good image quality.
The operating parameter of the X-ray source which is adjusted can be the X-ray tube current. In order to achieve the target pixel noise for the slice image, the exposure controller automatically adjusts the X-ray tube current once per each half-revolution in the scan. The tube current required for the target pixel noise is calculated based on the X-ray absorption profile during the previous half-revolution, and is extrapolated for the next half-revolution. The control of the tube current is undertaken by taking into account the delay associated with the X-ray tube response, and the tube current is set by means of a prediction so that the desired X-ray intensity is reached at the correct time, taking the delay into account. This prediction is required to compensate for the slow response of conventional X-ray tubes. A suitable method and apparatus for modulating a parameter of an X-ray tube, taking the modulation speed of the X-ray tube into account, are disclosed in U.S. Pat. No. 6,507,639, the teachings of which are incorporated herein by reference.
As supported by analysis of patient data, the attenuation profile of a patient does not change significantly between successive half-revolutions, and therefore the aforementioned extrapolation of the attenuation profile can be undertaken with acceptable accuracy. A software algorithm evaluates the attenuation profile of the previous half-revolution, and calculates the operating parameter, such as the X-ray tube current, appropriate for the exposure in the next half-revolution. During the first half-revolution in the scan, the software algorithm xe2x80x9cleamsxe2x80x9d the actual attenuation profile and sets the tube current to a nominal value based thereon. The algorithm also samples and integrates the attenuation profile of the half-revolution currently being scanned, and processes this information to calculate and predict the tube current necessary to achieve the target noise for the next half-revolution.
Moreover, the patient attenuation profile does not change significantly between successive projections (i.e., successive revolutions), and therefore alternatively the exposure controller can evaluate only every Nth projection in order to calculate the extrapolated attenuation profile, where N greater than 1.
Moreover, within a single projection it may be sufficient to detect only the maximum average attenuation that dominates the total sum over the projection, instead of making a calculation for each ray position within the projection.
The automatic exposure method and apparatus described herein can be combined with known techniques for modulating the dose as a function of the exposure angle within a single revolution, as described in U.S. Pat. No. 5,867,555.